Field ion microscope

The invention:Amicroscope that uses ions formed in high-voltage
electric fields to view atoms on metal surfaces.
The people behind the invention:
Erwin Wilhelm Müller (1911-1977), a physicist, engineer, and
research professor
J. Robert Oppenheimer (1904-1967), an American physicist
To See Beneath the Surface
In the early twentieth century, developments in physics, especially
quantum mechanics, paved the way for the application of
new theoretical and experimental knowledge to the problem of
viewing the atomic structure of metal surfaces. Of primary importance
were American physicist George Gamow’s 1928 theoretical
explanation of the field emission of electrons by quantum mechanical
means and J. Robert Oppenheimer’s 1928 prediction of the
quantum mechanical ionization of hydrogen in a strong electric
field.
In 1936, ErwinWilhelm Müller developed his field emission microscope,
the first in a series of instruments that would exploit
these developments. It was to be the first instrument to view
atomic structures—although not the individual atoms themselves—
directly. Müller’s subsequent field ion microscope utilized the
same basic concepts used in the field emission microscope yet
proved to be a much more powerful and versatile instrument. By
1956, Müller’s invention allowed him to view the crystal lattice
structure of metals in atomic detail; it actually showed the constituent
atoms.
The field emission and field ion microscopes make it possible to
view the atomic surface structures of metals on fluorescent screens.
The field ion microscope is the direct descendant of the field emission
microscope. In the case of the field emission microscope, the
images are projected by electrons emitted directly from the tip of a
metal needle, which constitutes the specimen under investigation.These electrons produce an image of the atomic lattice structure of
the needle’s surface. The needle serves as the electron-donating
electrode in a vacuum tube, also known as the “cathode.” Afluorescent
screen that serves as the electron-receiving electrode, or “anode,”
is placed opposite the needle. When sufficient electrical voltage
is applied across the cathode and anode, the needle tip emits
electrons, which strike the screen. The image produced on the
screen is a projection of the electron source—the needle surface’s
atomic lattice structure.
Müller studied the effect of needle shape on the performance of
the microscope throughout much of 1937. When the needles had
been properly shaped, Müller was able to realize magnifications of
up to 1 million times. This magnification allowed Müller to view
what he called “maps” of the atomic crystal structure of metals,
since the needles were so small that they were often composed of
only one simple crystal of the material. While the magnification
may have been great, however, the resolution of the instrument was
severely limited by the physics of emitted electrons, which caused
the images Müller obtained to be blurred.
Improving the View
In 1943, while working in Berlin, Müller realized that the resolution
of the field emission microscope was limited by two factors.
The electron velocity, a particle property, was extremely high and
uncontrollably random, causing the micrographic images to be
blurred. In addition, the electrons had an unsatisfactorily high wavelength.
When Müller combined these two factors, he was able to determine
that the field emission microscope could never depict single
atoms; it was a physical impossibility for it to distinguish one
atom from another.
By 1951, this limitation led him to develop the technology behind
the field ion microscope. In 1952, Müller moved to the United States
and founded the Pennsylvania State University Field Emission Laboratory.
He perfected the field ion microscope between 1952 and
1956.
The field ion microscope utilized positive ions instead of electrons
to create the atomic surface images on the fluorescent screen.When an easily ionized gas—at first hydrogen, but usually helium,
neon, or argon—was introduced into the evacuated tube, the emitted
electrons ionized the gas atoms, creating a stream of positively
charged particles, much as Oppenheimer had predicted in 1928.
Müller’s use of positive ions circumvented one of the resolution
problems inherent in the use of imaging electrons. Like the electrons,
however, the positive ions traversed the tube with unpredictably random velocities. Müller eliminated this problem by cryogenically
cooling the needle tip with a supercooled liquefied gas such as
nitrogen or hydrogen.
By 1956, Müller had perfected the means of supplying imaging
positive ions by filling the vacuum tube with an extremely small
quantity of an inert gas such as helium, neon, or argon. By using
such a gas, Müller was assured that no chemical reaction would occur
between the needle tip and the gas; any such reaction would alter
the surface atomic structure of the needle and thus alter the resulting
microscopic image. The imaging ions allowed the field ion
microscope to image the emitter surface to a resolution of between
two and three angstroms, making it ten times more accurate than its
close relative, the field emission microscope.
Consequences
The immediate impact of the field ion microscope was its influence
on the study of metallic surfaces. It is a well-known fact of materials
science that the physical properties of metals are influenced
by the imperfections in their constituent lattice structures. It was not
possible to view the atomic structure of the lattice, and thus the finest
detail of any imperfection, until the field ion microscope was developed.
The field ion microscope is the only instrument powerful
enough to view the structural flaws of metal specimens in atomic
detail.
Although the instrument may be extremely powerful, the extremely
large electrical fields required in the imaging process preclude
the instrument’s application to all but the heartiest of metallic
specimens. The field strength of 500 million volts per centimeter
exerts an average stress on metal specimens in the range of almost
1 ton per square millimeter. Metals such as iron and platinum can
withstand this strain because of the shape of the needles into which
they are formed. Yet this limitation of the instrument makes it extremely
difficult to examine biological materials, which cannot withstand
the amount of stress that metals can. Apractical by-product in
the study of field ionization—field evaporation—eventually permitted
scientists to view large biological molecules.
Field evaporation also allowed surface scientists to view the atomic structures of biological molecules. By embedding molecules
such as phthalocyanine within the metal needle, scientists have
been able to view the atomic structures of large biological molecules
by field evaporating much of the surrounding metal until the biological
material remains at the needle’s surface.

Electron microscope

The invention: 



A device for viewing extremely small objects that

uses electron beams and “electron lenses” instead of the light

rays and optical lenses used by ordinary microscopes.



The people behind the invention:



Ernst Ruska (1906-1988), a German engineer, researcher, and

inventor who shared the 1986 Nobel Prize in Physics

Hans Busch (1884-1973), a German physicist

Max Knoll (1897-1969), a German engineer and professor

Louis de Broglie (1892-1987), a French physicist who won the

1929 Nobel Prize in Physics











Reaching the Limit





The first electron microscope was constructed by Ernst Ruska

and Max Knoll in 1931. Scientists who look into the microscopic

world always demand microscopes of higher and higher resolution

(resolution is the ability of an optical instrument to distinguish

closely spaced objects). As early as 1834, George Airy, the eminent

British astronomer, theorized that there should be a natural limit to

the resolution of optical microscopes. In 1873, two Germans, Ernst

Abbe, cofounder of the Karl Zeiss Optical Works at Jena, and Hermann

von Helmholtz, the famous physicist and philosopher, independently

published papers on this issue. Both arrived at the same

conclusion as Airy: Light is limited by the size of its wavelength.

Specifically, light cannot resolve smaller than one-half the height of

its wavelength.

One solution to this limitation was to experiment with light, or

electromagnetic radiation, or shorter and shorter wavelengths.

At the beginning of the twentieth century, Joseph Edwin Barnard

experimented on microscopes using ultraviolet light. Such instruments,

however, only modestly improved the resolution. In

1912, German physicist Max von Laue considered using X rays.

At the time, however, it was hard to turn “X-ray microscopy” into

a physical reality. The wavelengths of X rays are exceedingly

short, but for the most part they are used to penetrate matter, not

to illuminate objects. It appeared that microscopes had reached

their limit.





Matter Waves



In a new microscopy, then, light—even electromagnetic radiation

in general—as the medium that traditionally carried image information,

had to be replaced by a new medium. In 1924, French

theoretical physicist Louis de Broglie advanced a startling hypothesis:

Matter on the scale of subatomic particles possesses wave

characteristics. De Broglie also concluded that the speed of lowmass

subatomic particles, such as electrons, is related to wavelength.

Specifically, higher speeds correspond to shorter wavelengths.

When Knoll and Ruska built the first electron microscope in 1931,

they had never heard about de Broglie’s “matter wave.” Ruska recollected

that when, in 1932, he and Knoll first learned about de

Broglie’s idea, he realized that those matter waves would have to be

many times shorter in wavelength than light waves.

The core component of the new instrument was the electron

beam, or “cathode ray,” as it was usually called then. The cathoderay

tube was invented in 1857 and was the source of a number of

discoveries, including X rays. In 1896, Olaf Kristian Birkeland, a

Norwegian scientist, after experimenting with the effect of parallel

magnetic fields on the electron beam of the cathode-ray tube, concluded

that cathode rays that are concentrated on a focal point by

means of a magnet are as effective as parallel light rays that are concentrated

by means of a lens.

From around 1910, German physicist Hans Busch was the leading

researcher in the field. In 1926, he published his theory on the

trajectories of electrons in magnetic fields. His conclusions confirmed

and expanded upon those of Birkeland. As a result, Busch

has been recognized as the founder of a new field later known

as “electron optics.” His theoretical study showed, among other

things, that the analogy between light and lenses on the one hand,

and electron beams and electromagnetic lenses, on the other hand,

was accurate.

Beginning in 1928, Ruska, as a graduate student at the Berlin Institute

of Technology, worked on refining Busch’s work. He found

that the energy of the electrons in the beam was not uniform. This

nonuniformity meant that the images of microscopic objects would

ultimately be fuzzy. Knoll and Ruska were able to work from the

recognition of this problem to the design and materialization of a

concentrated electron “writing spot” and to the actual construction

of the electron microscope. By April, 1931, they had established a

technological landmark with the “first constructional realization of

an electron microscope.”





Impact



The world’s first electron microscope, which took its first photographic

record on April 7, 1931, was rudimentary. Its two-stage total

magnification was only sixteen times larger than the sample. Since

Ruska and Knoll’s creation, however, progress in electron microscopy

has been spectacular. Such an achievement is one of the prominent

examples that illustrate the historically unprecedented pace of

science and technology in the twentieth century.

In 1935, for the first time, the electron microscope surpassed

the optical microscope in resolution. The problem of damaging

the specimen by the heating effects of the electron beam proved

to be more difficult to resolve. In 1937, a team at the University of

Toronto constructed the first generally usable electron microscope.

In 1942, a group headed by James Hillier at the Radio Corporation

of America produced commercial transmission electron

microscopes. In 1939 and 1940, research papers on electron microscopes

began to appear in Sweden, Canada, the United States,

and Japan; from 1944 to 1947, papers appeared in Switzerland,

France, the Soviet Union, The Netherlands, and England. Following

research work in laboratories, commercial transmission electron

microscopes using magnetic lenses with short focal lengths

also appeared in these countries.











Ernst Ruska





Ernst August Friedrich Ruska was born in 1906 in Heidelberg

to Professor Julius Ruska and his wife, Elisabeth. In 1925

he left home for the Technical College of Munich, moving two

years later to the Technical College of Berlin and gaining practical

training at nearby Siemens and Halsk Limited. During his

university days he became interested in vacuum tube technology

and worked at the Institute of High Voltage, participating

in the development of a high performance cathode ray oscilloscope.

His interests also lay with the theory and application of electron

optics. In 1929, as part of his graduate work, Ruska published

a proof of Hans Busch’s theory explaining possible lenslike

effects of a magnetic field on an electron stream, which led

to the invention of the polschuh lens. It formed the core of the

electron microscope that Ruska built with his mentor, Max

Kroll, in 1931.

Ruska completed his doctoral studies in 1934, but he had already

found work in industry, believing that further technical

development of electron microscopes was beyond the means of

university laboratories. He worked for Fernseh Limited from

1933 to 1937 and for Siemens from 1937 to 1955. Following

World War II he helped set up the Institute of Electron Optics

and worked in the Faculty of Medicine and Biology of the German

Academy of Sciences. He joined the Fritz Haber Institute

of the Max Planck Society in Berlin in 1949 and took over as director

of its Institute for Electron Microscopy in 1955, keeping

the position until he retired in 1974.

His life-long work with electron microscopy earned Ruska

half of the 1986 Nobel Prize in Physics. He died two years later.

To honor his memory, European manufacturers of electron microscopes

instituted the Ernst Ruska Prizes, one for researchers

of materials and optics and one for biomedical researchers.



See also: Cyclotron; Field ion microscope; Geiger counter; Massspectrograph;

Neutrino detector; Scanning tunneling microscope;Synchrocyclotron;

Electron microscope.





Further Reading